I think the debate of whether something will be considered a classic car has a lot to do with your definition of classic and collectible.

Here is a test of whether the car becomes classic or collectible.

TEST 1: The cars fall into one of the following categories:
CATEGORY 1: Cars with a following because they are enjoyable to drive, stylish, were groundbreaking or popular when new, or are quirky and unique. They were mass produced as opposed to hand built exotic or super cars. They can be broken down into;
CATEGORY 1A: Mass produced super cars e.g Porsche 944 Turbo, Porsche 911, Lotus
CATEGORY 1B: Convertibles and coupes e.g: Mercedes SLKs, BMW Z roadster, Porsche Boxter, Jaguar XJS, Honda S2000
CATEGORY 1C: Sedans: e.g Subaru WRX, Mercedes W123, W124, W114, W108, Volvo 144, Volvo 244, Bentley, Rolls, VW Beetle
CATEGORY 1D: Wagons and off roaders: e.g Early aluminium bodies Land Rovers, Toyota Landcruisers, Mercedes W123 Wagons, Volvo 244 Wagons

CATEGORY 2: Cars with a following because they are enjoyable to drive, stylish, were groundbreaking or popular when new, or were quirky and unique and are relatively rarer versions of a mass produced models made in very small numbers. , Mercedes AMG, BMW M, large engined variants of Category 1 cars e.g Mercedes 560SEC, SLK320, SLK350. They could be categorised as
CATEGORY 2A: Porsche 911 Turbo, Honda NSX, Smaller Ferraris,
CATEGORY 2B: Mercedes 560SEC, SLK320, SLK350, Mazda MX-5 NB SE, Mercedes AMG, BMW M
CATEGORY 2C: Bentley Mulsanne Turbo, Mercedes AMG, BMW M

CATEGORY 3: Cars with a following because they are enjoyable to drive, stylish, were groundbreaking or popular when new, or are quirky and unique. They were hand built exotic or super cars e.g Bugatti, McLarens, Jaguar XJ220, Porsche 959, Ferrari F40, Lamborghinis, Aston Martins etc. Examples of cars in this category that do not follow the trend are Rolls Royce, Maserati Sedans etc.

TEST 2: Reliability and Reasonable Maintenance Costs
With the exception of Category 3 for a car to become a popular collectible and classic they have to be reasonably reliable and not prohibitively expensive to maintain. Otherwise the car loses its desirability. They have to maintain that practicality, reasonable cost through the new to classic life cycle . E.g BMW 850i, early Jaguar XJS V12, some Rolls Royce models, Lancia, some Alfas and Maseratis.
A sufficient number of them need to survive to an old age without seriously rusting or requiring prohibitive maintenance.

TEST 3: Can stay fashionable and relevant or can make the transition from old fashioned to cool e.g steel bumpered Mercedes, Boxy old Volvos, some SAABs. They are able to stand out in a modern fleet of cars in around twenty or twenty five years and make people want them.

TEST 4: Has a loyal following, forums, websites, clubs, aftermarket suppliers dedicated to that model so there is a continued supply of parts

TEST 5: Can make it through the Life Cycle of a Classic

Its sold as a shiny new car with a warranty,
It is regularly serviced
Some change ownership
The warranty stops,
many crash,
It is replaced with a new model,
The value drops as it is no longer the new model
More change ownership
Mileages accumulate
Many wear out from high mileage and skipped maintenance
The value drops further and the price to repair becomes a bigger and bigger proportion of the value.
Its replacement is replaced so its not even the recent model.
Many stop being maintained correctly. They drop out of the pool.
Insurance companies begin writing more and more off as even small bingles become uneconomic to repair.
The price becomes far more affordable to people who could never have contemplated buying them.
This new breed of owners either begin taking a lot of care or running the car to the ground.
A new model comes out again, by this time the numbers are dwindling,
The car is no longer cool or desirable except to dedicated followers, the numbers of good cars crash. it disappears from the general public's consciousness.
The price stabilises and stops falling. At this point the high number of used parts keeps enthusiasts happy
Five or so years pass.
They become so rare on the road that people start taking notice.
They begin to qualify for reduced registration costs as interesting collectible, classic etc
Those looking for an old interesting classic begin investigating them through enthusiasts who keep the flame alive.
They enter the market and begin driving up prices to a point where supply meets demand.
Some of them appear in ads or movies (this adds to their appeal and hence price)

Factors that help the eventual price:
Originality
Colour
Low Ks
Highly optioned
Larger Engine sizes
The original exclusivity and rarity
The original price
Whether they are featured in popular culture e.g movies etc.
Convertibles and coupes will generally always be more expensive than sedans
The Prices they will command will depend on the original category they sit in and their age. Some cars in Category 4 will probably be closer to blue chip investments. Very very few of them might end up costing more than the new price of the existing equivalent model. Most will do well to end up at a value equal to a 3 year old current equivalent model.

You have obviously given that a lot of thought! I think it is simpler than that and comes down to supply and demand. Notes on desirability are important to create that demand but also it can be the car a lot of people relate to from their past, but main point is lots of people want one. Then it comes down to how many are available. Along those lines you would think an MX5 Mazda should be a collectable classic, and i guess people are collecting them, but they are still cheap as chips just due to the sheer volume of them. Availability of cheap parts might keep more on the road too. R170 may have better chance due to less of them (still lots though) and more expensive repairs, but then I reckon less boys had a poster on their wall of them when younger etc. Using another example of that, how many W116 6.9's vs gt falcons and Monaros? How do their prices compare? Comes down to a lot more demand even though 6.9 rarity.

You make a good point about this car not being on the wall of teenagers but it certainly was desired by young professionals who could not afford it.

They are Mercedes and convertibles and that will always make them special. I just bought an SLK320 and a few friends, admittedly not car people, said they thought I would have paid $80,000 for it. So you can see the mystique still exists.

none of that surprises me. Anyone who gets in my 560SEL raves about the limousine ride and good looks. They like it but not enough to buy it and not enough people to raise demand to a point to outrun supply and then increase prices. Classic yes, and widely recognised as such. The point that frustrates me is the values/investment returns going through the roof on 'lesser' cars. So was just making the point about all the comments about the car itself, when surely it is about supply and demand when local products sell for multiples of what older Benzes do. But I hope lots of those young professionals want to tuck a nice condition classic one away one day and drive up that demand.

Same goes of parts - trying to sell some old Benz bits on ebay and getting zero response but a rusty set of very dented FJ holden bumper overiders fetch $150! other odds and sods of holden parts sold straight off and for much more than I would think equivalent benz parts would sell for.

Great point! I looked at them from time to time to see how the values of used ones are going. Quick look then and 10yo good low mileage cars look like they can be had for $40k. Surely that is good buying and they won't drop much from there? Do they have any inherent nasty problems from that era?